This invention relates generally to optical sighting systems and more particularly to such systems where a common aperture is simultaneously shared by a plurality of optical scanners.
Parameters which are given considerable attention in the design of multi-function airborne electro-optical systems are, package size and the alignment and stability of the optical axes. A large diameter package can adversely affect the aircraft performance while an error in alignment or stability reduces the performance of the electro-optical system.
The diameter, and to some extent, the length of an optical system is established based upon the required system performance parameters and the available sensor or illuminator parameters. Once these parameters have been selected, there is little that can be done to reduce the aperture or total volume of the optics for a single function. Volumetric efficiency of the optical system can, however, be improved for multi-function optical systems by allowing the subsystems to share common apertures and elements.
The need to share common elements is reinforced by the requirements for highly accurate line-of-sight stabilization and alignment. As a minimum, it is desirable to utilize a common line-of-sight stabilization and pointing gimbal in order to reduce the alignment errors contributed by this component to zero and to reduce volume and complexity by eliminating the necessity for duplicate gimbals. The use of a common gimbal implies a common window.
Improvements in the overall stability and maximum diameter can be gained by utilizing additional elements such as objective lenses, filters and relays depending on the application. The reduction in diameter also results in the reduced drag and buffeting in the air stream; this improves both aircraft performance capability and image stabilization.
Sharing of common apertures has long been a practice for microscopes which use a beamsplitter to provide illumination through the objective and for multi-spectral systems which use spectral beamsplitter or filter arrangements to generate images in different spectral bands formed by a common objective. However, in the case of optical systems which are designed to provide both far field scene illumination from high energy sources such as lasers and far field imaging at low signal levels, it has been standard practice to design physically separated optical subsystems for each function. This is done primarily to prevent field-of-view cross-over in the near field and thus to reduce the degrading effects of scatter from near field objects such as common windows, mirrors, lens elements, and the atmosphere.